This invention relates to thermoplastic elastomer blends of elastomeric block copolymers and certain olefin copolymers and, more specifically, to the fabricated films of the blends which exhibit high tensile strength and elasticity.
Thermoplastic elastomers are readily commercially available and are fabricated as elastomeric films in products such as disposable diapers, leggins, waistbands and gloves. Typically, these elastomeric films have been made of elastomeric block copolymers, combinations of elastomeric block copolymers and liquid butadienes, combinations of elastomeric block copolymers and ethylene vinyl acetate copolymers, thermoplastic urethanes, ethylene-propylene rubbers (EP or EPR), including ethylene-propylene diene terpolymers (EPDM), natural rubbers and combinations of polyester copolymers and ethylene-vinyl acetate copolymers. Each of these materials has suitable properties for elastomeric films, such as low tensile set, low modulus, and high elongation, but each also has acknowledged deficiencies. For example, elastomeric films containing thermoplastic urethanes, copolyesters and/or ethylene vinyl acetate copolymers are expensive due to the cost of raw materials. The tensile set and modulus of films made of these well known elastomers are acceptable for many applications, but even greater advantages would be realized from films fabricated from elastomeric materials with lower tensile set and lower modulus.
It is well known to blend thermoplastic polymers in varying proportions with thermoplastic rubbers. Thermoplastic polymers such as polypropylene (meaning isotactic or crystalline polypropylene), polystyrene, polyethylene, ethylene-vinyl acetate copolymer, and polyurethane have been blended with thermoplastic rubber to effect compositions having varied thermoplastic and/or elastomeric properties.
The present invention relates to thermoplastic elastomer films made of elastomeric block copolymers and polyethylene/x-olefin copolymers. The relevant prior art to same includes the following:
A) U.S. Pat. No. 4,476,180 describes elastomeric films comprising from about 40% to about 80% of a thermoplastic block copolymer of styrene-butadiene-styrene (S-B-S) and from about 20 to about 60% of an ethylene/vinyl acetate copolymer (EVA), the fabricated films of which demonstrate good tensile strength and elasticity. While these films demonstrate suitable properties for most elastomeric film applications, the shortcomings of these films are (1) a significantly reduced elasticity of the patented blend as compared to the elastic property of the pure elastic S-B-S copolymer; (2) difficulty in processing of the blend; and (3) typically excessive blocking of the S-B-S elastomeric copolymer. U.S. Pat. No. 4,977,014 attempted reduction of these shortcomings by adding polystyrene to the ""180 blend composition of the ""180 patent.
B) Japanese patent application No. 554-120646 published on Sep. 19, 1979 discloses a composition of a styrene-butadiene elastomeric copolymer and ethylene/alphaolefin copolymer, the sole use of which composition was ineffective in film formation. This composition was rendered film-forming when a high styrenic styrenebutadiene copolymer was used as a third component. The ethylene/xcex1-olefin copolymer has a density of 0.87 to 0.90, and is of low crystallinity.
While all of the cited elastomeric blend film art demonstrates certain advantages relating to processability, or physical characteristics such as softness, tensile strength or elasticity, they all fail to teach or appreciate the use of elastomeric block copolymer and linear low density polyethylene (xe2x80x9cLLDPExe2x80x9d) blends as demonstrated in the present invention. Consequently, the prior art elastomer/polyolefin blends retain certain deficiencies relating to film fabrication for apparel or healthcare use.
As any garments or healthcare products are worn, the elastic polymers used therein must have enough elastic strength to hold any of these items snugly in place. Conversely, an unworn garment or item must have a high degree of elastic recoverability so that the elastomeric polymer used therein returns essentially to an original shape during non-use. Additionally, any elastomeric films used in apparel items must also demonstrate a high degree of repeatability as any such items are worn over a long period of time. These conditions and constraints dictate the suitability of those elastomeric compositions of the prior art which can be fabricated into articles for garment or wear use.
As indicated above, typical elastic materials utilized for clothing or healthcare applications include the thermoplastic elastomeric films of U.S. Pat. No. 4,476,180 which contain elastomeric block copolymers of monoalkenyl arenes (styrene) and conjugated dienes (butadiene). These films generally exhibit good strength and elastic properties but also undesirable excessive blocking. Blocking is the tendency of a film to adhere to itself. When films made from block copolymers of monoalkenyl arenes and conjugated dienes are stored on a roll or in stacks, the film layers become difficult to unroll or unstack over a period of time due to their blocking tendency. Such blocking tendency can cause the film to tear when it is being unwound from a roll or being unstacked. To avoid such blocking it is generally necessary to add significant quantities of slip and anti-block agents to the film blend. The addition of these materials is undesirable because such material can accumulate on the film fabrication apparatus, particularly the rollers, and this accumulation can result in significant down time for machine cleaning.
It now has been found that the addition of certain LLDPE copolymers to elastomeric block copolymers of monoalkenyl arenes and conjugated dienes results in films having substantially reduced blocking tendency while exhibiting optimum elastic properties.
The instant invention is a thermoplastic elastomeric film comprising (a) from about 20% to 70% of an elastomeric block copolymer having the general configuration:
Axe2x80x94Bxe2x80x94A 
wherein the polymeric blocks A comprise monoalkenyl arene polymer end blocks and block B comprising an elastomeric conjugated diene polymer mid-block, the blocks A comprising from about 8 to 55% by weight of the block copolymer having a melt index of less than about 100; and (b) from about 20% to 70% of an LLDPE xe2x80x9cplastomerxe2x80x9d (as defined hereinafter) copolymer having less than 35% ethylene crystallinity. Particular LLDPE plastomer copolymers for use in the present blends include linear low density ethylene/a-olefin copolymers having densities from about 0.86 to 0.90 g/cm3, molecular weight distributions of less than 6 and melt indices of between 0.2-1000 g/cm3.
The LLDPE plastomer copolymers to be used in accordance with the present invention have very low ethylene crystallinity. The films made from blends of these low crystalline ethylene copolymers and elastomeric copolymers have the characteristics of high elasticity and low blocking tendency. These benefits occur because the instant film blends contain low crystalline plastomeric ethylene copolymers having certain characteristics of molecular weight distributions (Mw/Mn) -(MWD), melt index (MI), density, and crystallinity.
The films of the present invention have improved elastic recovery over those films of the prior art and provide benefits and applications where both low modulus and excellent elastic recovery are necessary as in the case of elastic components of disposable diapers. The instant films exhibit a low blocking tendency and consequently can be manufactured more efficiently and less expensively than other elastomeric films which require significantly more slip and antiblocking additives for their processed blends.
The elastomeric films of this invention are films of about 0.5 to 15 mils in thickness and may be produced by any well known film manufacturing processes such as these disclosed by J. H. Briston and L. L. Katan in Plastic Films, published by John Wiley and Sons, New York (2nd ed. 1983), herein incorporated by reference. Commonly known methods of producing film which may be utilized in the present invention include blowing, casting (extrusion and solvent), calendaring and extrusion methods, such as blow extrusion or slit die extrusion and cast embossed.
Briefly, the present invention is directed to a film comprising from about 20% to 70% of a monoalkenyl arene diene elastomeric block copolymer and from about 30 to 80% of a linear low density plastomeric polyethylene (LLDPE) copolymer having a density in the range of from 0.86 to 0.90, a melt index of from 0.2 dg/min to 1000 dg/min, an MWD in the range of less than 6, and a CDBI greater than about 45%.
A process for producing the elastomeric film of this invention comprises the steps of (a) providing a thermoplastic elastomeric film blend of (1) a block copolymer elastomer and (2) an LLDPE having a density of from about 0.86 to 0.900 g/cm3, a MWD in the range of 2 to about 3.5, a melt index of less than 1000 dg/min, and an ethylene crystallinity of less than 35%; (b) extruding the blend composition through a die having a gap greater than about 5 mils and less than about 120 mils; (c) expanding the extruded film to a thickness of between 0.5 mils and about 15 mils by application of differential pressure as exerted by a gas and (d) rapidly cooling the expanded extruded film.